In a wireless communications system, the communications system's capacity may be significantly improved when a first device has full or partial knowledge of a channel (e.g., channel information) over which it will be transmitting. The channel information may be referred to as channel state information (CSI). CSI may be obtained by the first device over a reverse feedback channel, where a second device that receives transmissions made by the first device transmits CSI to the first device over the reverse feedback channel.
Generally, communications in a communications system occur over uni-directional links. A first uni-directional link may be referred to as a downlink (DL) that originates at a communications controller (also commonly referred to as a base station, a NodeB, an enhanced NodeB (eNB), and so on) and ends at a communications device (also commonly referred to as a mobile station, a terminal, a subscriber, a User Equipment (UE), and so forth). A second uni-directional link may be referred to as an uplink (UL) that originates at the communications device and terminates at the communications controller. In the DL, the communications controller may be the first device and the communications device may be the second device.
The capacity and coverage of the wireless communication system can be significantly improved by using multiple antennas at transmitter and/or receiver. Such wireless communications systems are referred to as Multiple Input Multiple Output (MIMO) systems and can exploit the spatial dimension of the communication channel to transmit several parallel information carrying signals, commonly referred to as spatial multiplexing. The additional gains can be achieved by adaptation of a number of simultaneously transmitted information carrying signals to a level that the channel can support, which is commonly referred to as transmission rank adaptation.
Additional gain can be also obtained by using precoding that adjusts the phase and amplitude of the signals to better fit current channel conditions. The aforementioned signals form a vector-valued signal and the adjustment operation can be implemented as multiplication with a precoding matrix. Based on information related to channel conditions, the precoding matrix can be chosen from a finite and countable set, a so-called codebook. A different codebook can be defined for different transmission rank and the precoding matrix can be indexed by a precoding matrix indicator from the corresponding codebook.